This invention is in the field of sewing machines; more particularly, it is concerned with a low thread supply alarm for the lower thread of the lockstitch sewing machine.
Low bobbin thread supply alarms are known in the prior art. There are, for example, the devices disclosed in U.S. Pat. Nos. 3,991,692 of Papajewski et al, 3,601,073 of Simpson and 3,129,680 of Doerner. The first two patents refer to devices wherein bobbin collapse due to thread depletion is detected, while the patent of Doerner relates to use of a magnet supported adjacent a bobbin hub, which magnet is released on thread depletion to move to a position where it can actuate a Reed switch in order to provide a suitable alarm.
It is also known in the prior art to intermittently sense the condition of the bobbin thread supply in order to determine, relatively continuously, the status thereof. Such a device is shown in the Japanese Patent Publication No. 17586/72 by Ozaki, published on May 23, 1972.
It is further known in the prior art to utilize magnet sensors, for which see the Japanese Utility Model Publication No. 6913/73 of Gotoh, published on Feb. 22, 1973.
The above referred to devices are generally not ideally suited to practical use. Those devices disclosed by the U.S. patents require extra manipulation of the bobbin prior to winding, which makes their use relatively inconvenient. The devices disclosed by the Japanese patent publication of Ozaki is relatively cumbersome, and the U.M. pulication of Gotoh discloses an arrangement whose use requires an especially prepared thread supply for each bobbin.
What is required is a compact device, ideally using standard, trouble free, components, which require no special preparation for their use.